Please explain the meaning of the following quote from The Road by Cormac McCarthy:
You forget somethings, don’t you?
Yes. You forget what you want to remember and you remember what you want to forget.
This quote comes from towards the beginning of the novel and is a conversation between the father and the son whilst they walk through a decaying and destroyed city, they see a corpse that is "dried to leather" and "Grimacing at the day." Following this sight, the father gives his son the following piece of advice:
Just remember that the tings you put into your head are there forever, he said. You might want to think about that.
The rest of the quote indicated in this question comes after this. The quote is important as it reveals important information about the kind of world that the father and son live in. This is a world where discomforting sights and scenes of death, horror and pain are commonplace, and where the boy is obviously confronted with such sights very frequently. This explains his hopeful question to his father, as he wants to know that he will be able to eventually forget some of the dreadful sights he has witnessed. However, the father responds with a kind of truism that points towards the grim kind of life that humanity is now forced to eke out in this dystopian world: the only things that humans forget are what they want to remember, and it is the terrible sights and experiences that brand themselves into the brain and are impossible to forget. Such a bleak and depressing comment sets the tone for the rest of the novel.